ACF does not automatically update relationships on the other end, that is what https://www.advancedcustomfields.com/resources/bidirectional-relationships/ addresses. Alternately you can use this plugin https://wordpress.org/plugins/post-2-post-for-acf/.
By using the above code or the plugin you remove the need to do a reverse relationship query as described in the first document https://www.advancedcustomfields.com/resources/querying-relationship-fields/
